Contribution à l'étude de virus de mollusques marins apparentés ...

Introduction
The Pacific oyster, 'Crassostrea gigas, is the most important species of molluscs reared in the world
(FAO, 1989), and is of particular importance for French mariculture. However, cultivation may be
endangered by the occurrence of epizootics, especially viral diseases, which are considered one of the
major risks to production. Indeed, oyster mortalities were described and associated with the presence
of several families of morphologically described viruses among different species of ostreids. The first
was reported in adult American oysters, Crassostrea virginica, and described as a member of the
family Herpesviridae (Farley, 1972). More remarkably, mass mortalities of adult Portuguese oysters,
Crassostrea angulata, among French live-stocks, were caused by an lridovirus between 1967 and
1970 (Comps and Duthoit, 1976; Comps et al., 1976; Comps and Bonami, 1977). Other important
viruses, described trom ostreids, are members of the families lridoviridae, Papovaviridae, Togaviridae,
Retroviridae and Reoviridae (Elston, 1979; Eiston and Wilkinson, 1985; Farley, 1976; Farley, 1978;
Meyers, 1979).
More recently, in 1991, other viruses belonging to the Herpesviridae, were associated with high
mortalities of hatchery-reared larvae of C. gigas in France (Nicolas et al., 1992) and in New Zealand
(Hine et al., 1992). Subsequently, sporadic high mortalities of larval C. gigas have been observed in
1992, 1993 and 1994 associated with a herpes-like virus, in some French hatcheries (Renault et al.,
1994b). The pathogenicity of this virus was demonstrated earlier by experimental transmission of the
disease to axenic C. gigas larvae (Le Deuff et al., 1994). Additionally, mortalities of oyster spat of O.
edulis and C. gigas were also associated with herpes-like viruses (Comps and Co'chennec, 1993;
Renault et al., 1994a). These herpes-associated mortalities among ostreids ail occurred du ring the
hotter summer periods. Thus, it appeared necessary to investigate the effects of tempe rature on the
induction of these viral infections. For this purpose, we cultured C. gigas larvae at high (25-26°C) or
low (22-23°C) temperatures and we examined the susceptibility to the disease of larvae Irom
broodstocks 01 different geographical origins.
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